The invention relates to a method and to a machine for removing material from axially spaced-apart portions of rotary workpieces which are symmetrical with reference to their axes of rotation. Typical examples of such workpieces are valving elements of lift valves, e.g., simple poppet valves of the type used in the engines of motor vehicles, wherein one end of the stem constitutes a foot and the other end of the stem carries a mushroom-shaped or otherwise configurated enlarged portion or head with a normally frustoconical circumferentially extending sealing surface which can engage the seat in the body of the valve wherein the valving element is put to use.
The treatment of workpieces of the above outlined character necessitates removal of material with utmost precision so as to ensure that the sealing surface of the head will be located at a predetermined distance from certain portions of the foot, e.g., from the end face of the foot and/or from a groove in the peripheral surface of the foot. Such high-precision treatment cannot be ensured in conventional grinding and like material removing machines wherein a workpiece is repeatedly clamped, first to allow for treatment of one of its end portions and thereupon to allow for removal of material from the other end portion. Valving elements which are used in many types of lift valves and analogous valves must be machined with tolerances which constitute minimal departures from optimum dimensions. Thus, if the valving element is a poppet with a head at one end and an externally grooved foot at the other end of the stem, the distance between the flat end face of the head and the groove, the distance between the central portion of the sealing surface on the head and the groove and/or the distance between the central portion of the sealing surface on the head and the end face of the foot must match, or can deviate only negligibly from, a preselected value which ensures most satisfactory operation of the valve. One of the main reasons for the failure of conventional grinding machines and grinding methods to ensure the removal of material from the head and from the foot with the presently expected and required degree of accuracy is the need for repeated clamping and unclamping of a workpiece prior to completion of material removal at both ends of the stem.